As a countermeasure to recent environmental problems, especially to air pollution caused by automobile exhaust gas and global warming stemming from carbon dioxide emissions, much attention is focused on fuel cell technology which achieves clean exhaust emissions and offers high energy efficiency. A fuel cell is an energy conversion system in which hydrogen or hydrogen-rich reformed gas as fuel is supplied together with air to an electrolyte/electrode catalyst composite to cause an electrochemical reaction so that chemical energy is converted to electrical energy. In particular, a solid polymer electrolyte fuel cell that uses a proton exchange membrane as an electrolyte is inexpensive and easily reducible in size, and further has high power density, therefore, applications to power sources of mobile objects such as automobiles is highly expected.
In this solid polymer electrolyte fuel cell, a proton exchange membrane functions as an ion conductive electrolyte when saturated with water, and also has a function of separating hydrogen and oxygen from each other. Low water content in the proton exchange membrane leads to increased ion resistance and causes mixing of hydrogen and oxygen, resulting in failed generation of electricity as a fuel cell. Meanwhile, when hydrogen ions separated at a hydrogen electrode pass through the proton exchange membrane by generation of electricity, water in the proton exchange membrane also moves, and therefore the proton exchange membrane on the hydrogen electrode side tends to dry. Furthermore, if the amount of water vapor contained in supplied hydrogen or air is small, the proton exchange membrane tends to dry near the inlets of the respective reaction gases.
From these reasons, the proton exchange membrane of the solid polymer electrolyte fuel cell needs to be positively humidified by supplying water thereto from outside, and hence some kind of humidification means is provided to humidify the electrolyte itself or to humidify hydrogen and air to be supplied. When water still remains in a fuel cell after system shutdown, however, it freezes depending on ambient temperatures, which may lead to difficulty in smooth starting of the system or to deterioration of the proton exchange membrane and the like. Accordingly, conditioning of the inside of a fuel cell is necessary at the time of system shutdown.
U.S. Pat. No. 6,635,370 and Japanese Patent Application Laid-Open No. 2002-313394 propose various methods for conditioning a fuel cell at system shutdown. According to U.S. Pat. No. 6,635,370, 0.0001% to 4.0% of hydrogen is maintained within a fuel cell at system shutdown in order to suppress deterioration of the proton exchange membrane which is caused at the time of system shutdown or during storage. Furthermore, according to Japanese Patent Application Laid-Open No. 2002-313394, a humidifier and a dehumidifier are provided to dry the membrane within a fuel cell by using the dehumidifier before system shutdown so that the fuel cell system can be started even below the freezing point.